Grain Filling and Bleaching - By: Tauqeer Hassan

Bleaching is done before staining, grain filling after. You might have deep marks to remove, such as those that water makes on oak (a particularly reactive timber), or you might want to lighten the whole surface generally, especially if you have used a caustic soda stripper.

Bleach works well on open-grained hardwoods like ash, beech or elm, but not on the close-grained ones like cherry, rosewood or padauk. Neutralizing bleaches, like strippers, is vital; they are corrosive and danger¬ous, so be careful - always add crystals or solutions to water, not the other way round. Sand with medium grit paper (100) to open the grain before you bleach - you'll be sanding with finer papers afterwards - and do not bleach veneers.

The weakest and easiest to use is household bleach. Start with a weak solution to see how it works, then strengthen it if you need to. Mop it on, leave it to dry, and rinse it thoroughly with clean water and vinegar. Oxalic acid is the next in strength, and is widely used in the trade. It is good for removing water-stains from oak. Use 50g (2oz.) of crystals per 500 ml (1 pint) of hot water, mop it on hot, leave it to dry, and neutralize it with a solution of 1 part ammonia to 10 parts water. Two-part 'A & B' solution (a mix of hydrogen peroxide and ammonia or caustic soda) is very strong and quite dangerous. Apply the first coat, leave it for about 10 or 20 minutes, then apply the second. Neutralize - stop the action if you want to - with full strength vinegar. Do not use this solution on oak.

Grain filling aids your smooth build up of finish - the glassy feel - and, if you put it on right, it will not obscure the grain pattern. It also surface-levels the pores, which would otherwise give your clear finish a pitted effect, however many coats you put on.

Proprietary fillers come in a wide range of colours, and should be used one shade darker than the wood, which itself will darken as if it was wet when the clear finish is applied. Thin them to a paste for the hard¬woods and a thick liquid for soft, open-grained pine, using the correct solvent - white spirit or meths. Rub in the filler with a coarse cloth across the grain, let it 'go off - begin to dry - and then rub off the surplus, also across the grain, packing it hard into the pores. Finish by wiping hard along the grain.

The thinner fillers should be pushed in with a brush. Leave them to dry for a day, then fine paper the surface down, and rub in raw linseed oil to clear the muddy appearance. Wipe the surplus off thoroughly.

You can mix your own filler with superfine plaster, french chalk, or silex (china clay). Use pigments or oil stains to colour it, and a solvent compatible with the colour and the top finish. This is especially important for cellulose, which has its own nitro fillers, and will not take to oils. There are shellac-based liquid fillers available for french polishing close-grained woods.

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